This invention relates to heavy duty off-the-road (OTR for brevity) tires of the type used on farm tractors and heavy construction vehicles and more particularly to an improved tread design which reduces vibration during high speed operation on hard surfaces.
Agricultural tractors, graders, log-skidders and other OTR vehicles normally experience severe vibrational problems when traveling on hard surfaces, such as paved highways or packed fields. The problem is largely due to the lugs which are arranged for effective OTR operation on soft ground.
The tread designs which have been preferred for OTR tires provide high traction in soft ground and expulsion of the earth from the transverse recesses between the deep lugs of the tread. Over the years many different lugged tires have been employed which meet these requirements and which provide the durability and long life needed for a commercial product. However, the commercial tire tread designs for lugged OTR tires have not provided optimum riding qualities on highway surfaces.
Because of the rigidity of the ordinary farm tractor and the absence of shock absorbing systems such as are used on passenger cars and other highway vehicles, it can be dangerous to operate such a tractor at substantial speeds, such as 20 to 25 miles per hour. This is particularly true when using conventional OTR tires with deep widely spaced lugs which cause severe pounding and vibration in contact with a hard highway surface. This is one important reason that farm tractors usually operate on the highway at slow speeds below 15 miles per hour.
At higher speeds the vibration caused by conventional OTR tires can become intolerable. When using widely spaced lugs, large radial forces are transferred to the wheel axle because of concentration of tire load at the tip of each lug as it moves into contact with the road. With the usual curved tread profile and a herringbone lug arrangement, the rolling radius of the tire decreases as the load is progressively transferred toward the outer side portions of the tread, and, when the next lug contacts the road, the rolling radius is suddenly increased. The cyclic motion produced in this manner results in large forces applied to the driven wheel axle in a vertical direction and also large tractive forces resisting rotation of the tire.
With the conventional lug arrangements the vibration problems are much more serious at high speeds due to the cyclic movement of the center of tire pressure from one side of the peripheral centerline of the tread to the other. Vibration tests show that such cyclic movement causes large forces and moments to be developed at the wheel axle and that these can be accentuated as the result of resonance effects at certain rotational speeds.
Many different lug arrangements have been proposed for OTR tires, and some of the tires have performed satisfactorily on highway surfaces, but the practical lug designs which have met with commercial acceptance for OTR use because of good traction and effective operation on soft ground have caused vibration problems when the tires are operated at high speeds on hard highway surfaces.
It is difficult to provide a tire suitable for highway use which meets the requirements of effective OTR use, particularly the requirements as to traction and earth removal. Tread designs have been proposed with a continuous circumferential rib at the crown but these are unacceptable for the driving wheels of a tractor because of the inability to provide adequate traction. Closely spaced lugs reduce the vibration problems but are unacceptable for OTR tires because of inadequate traction and inability to provide efficient removal of earth from between lugs. For these and other reasons, the commercial OTR tread designs have employed deep widely spaced lugs which are not well suited for high speed operation on paved highways.